Efforts to increase efficiency and improve environmental impacts of internal combustion engine have led to providing internal combustion engines with systems that treat the exhaust stream produced by such engines. Aftertreament exhaust systems may include a variety of components, such as filters and catalysts for reducing contaminants or targeted by-products in the exhaust. Some exhaust treatment systems include a reductant delivery device, for example an injector, placed in the exhaust stream. The reductant delivery device is placed in the exhaust stream of the aftertreatment system to deliver reductant, such as gaseous NH3 or urea containing liquid NH3, for NOx reduction. The reductant delivery device may include a lattice of tubes with varying diameter apertures or placement from the central axis. This is a passive device that delivers a pressurized stream of reductant from a source. The reductant delivery device is designed to maximize the distribution of the reductant in the cross-sectional area of the exhaust stream: The reductant delivery device may be designed to deliver a reductant to the exhaust to reduce or transfer certain components of the exhaust.
Changes in engine performance under certain conditions causes variability in the exhaust stream and warrants variation in the delivery of the reductant. However, passive reductant delivery devices may be designed for proper operation under specific constant conditions. Specifically, the mass flow of the reductant for treating the exhaust stream is dependent on the molar ratio of NH3 required to reduce NOx; therefore, it is not constant. However, when the reductant delivery device is designed, it must be designed for the maximum flow rate of reductant to prevent the apertures from throttling the release of the reductant into the gas stream. If this happens, then there will be a reductant deficit in the exhaust stream and NOx will not be adequately reduced.